FIG. 1A relating to the prior art is a schematic representation of a mass-manufactured injected thin part (100). For example, this part is a cover for an electronic equipment. When it is large, for example, if it is a cover for a large-screen television set, said part is made of a polymer filled with reinforcement in the form of fibers or balls. Such a part (100) generally comprises a side (110), called the finish side, smooth or with a textured finish, which side is visible when said part is integrated into the device. In one particular embodiment, this side comprises a decoration, which is obtained by injecting the plastic in the mold while a decorative film is set against the inside of said mold. The part (100) also comprises a side, called the technical side, which comprises many relief features such as ribs (121), screw holes (122), grooves (123) etc. In FIG. 1B, the part (100) is made by hot injection of plastic comprising reinforcing filler in the enclosed cavity (153) of a mold (150). The mold thus comprises a fixed part (151) and a mobile part (152). The injected material is brought into the cavity (153) by a conduit (161) made in the fixed part, which conduit is connected to an injection device (160), for example a screw. In order to fill the whole cavity (153) and ensure an even appearance, particularly of the finish side of the part, the mold (150) must be pre-heated prior to injection so as to obtain uniform temperature at the surface of the cavity in contact with the injected polymer. In the case of a mass manufacturing method, the time taken to pre-heat and cool the mold should be reduced as much as possible. According to prior art, the molding surfaces of the mold, particularly the molding surface located on the fixed part of said mold, are for instance heated by heating devices placed in grooves or bores under said molding surfaces. These arrangements are difficult to make on large molds and make the molding surfaces mechanically fragile. Heating is carried out by conduction inside the very parts of the mold, and so a large volume of material is heated, leading to high energy expenditure, making the achievement of a high heating speed difficult.
In order to increase the heating speed, in the prior art, the surfaces of the mold on each side of the cavity can be heated by induction.
In FIG. 2, such a pre-heating method has been described in the document WO 2010 046582. According to that method of the prior art, the two parts (151, 152) of the mold are made up of an electrically conductive and ferromagnetic material, for example steel comprising a large proportion of ferritic phase. Advantageously, each of these two parts is enclosed in a carcass (251, 252) made of non-magnetic conductive material, such as copper (Cu), with the exception of the surfaces (261, 262) called the molding surfaces, which demarcate the cavity of the mold. An induction circuit (210) made up of one or more coils, surrounds the two parts (151, 152) of the mold. An intermediate part (270), called the core, made up of electrically conductive material, is placed between the two parts (151, 152) of the mold. The core is electrically insulated from the two parts (151, 152) of the mold. Thus placed, the core (270) comprises surfaces (271, 272) separated by a small distance from the molding surfaces (261, 262) of the two parts of the mold, thus demarcating two insulating gaps (e1, e2) with said molding surfaces. When high-frequency alternating current flows in the induction circuit, induced currents flow on the sides (261, 271, 262, 272) of the core and the molding surfaces opposite, on each side of these gaps, leading to the rapid heating of the ferromagnetic molding surfaces (261, 262). The high magnetic permeability of ferromagnetic steel makes the induced currents flow only on the surface, over a small depth from said molding surfaces. Thus, this pre-heating method makes it possible to rapidly inductively heat the molding surfaces, by concentrating the heating on those surfaces, when the core (270) is interposed between them, with the mold open. To carry out the injection, the core is removed, the mold (150) is closed by bringing the two parts (151, 152) closer, the material in the liquid or pasty state is then injected in the molding cavity through one or more conduits (not shown). The pre-heating temperature is controlled by the electrical power delivered to the induction circuit and by the heating time. That temperature is selected to be just sufficient to allow the easy flow of injected material throughout the entire cavity, wherein the heat of said material is then removed through the mass of the mold, which advantageously comprises a cooling circuit, for example by means of the circulation of fluid in cooling circuits (281, 282) placed in each part of the mold and extending through them just below the molding surface. Thus, the possibility of only heating the molding surfaces over a small thickness allows productivity gains both for heating and cooling.
While this method of the prior art is effective for pre-heating the molding surface of the finish side of the part, it is however not suitable for pre-heating the molding surface of the technical side. Indeed, the many relief features and technical arrangements on that side, such as drawers or slides, do not make it possible to easily obtain a constant gap between the core and the corresponding molding side; furthermore, these shape features disrupt the flow of induced currents, producing local overheating or even electrical arc phenomena.
The document AT 504784 describes a method and device for pre-heating a mold adapted to an injection molding method for plastic, comprising a device for pre-heating one of the molding sides of the mold by thermal radiation. In order to obtain the rapid pre-heating of said molding surface, that molding surface is made up of the side of a thin part distant from the mold body when said mold is in the open position. Thus, the volume of material heated by radiation is smaller. However, this embodiment is complex and cannot be adapted to the molding surface corresponding to the technical side of the part.
The document DE 10 2008 060496 describes a pre-heating method and device consisting in transferring part of the mold comprising the molding surface outside said mold to pre-heat it.